1. Field of the Invention
The present invention relates to explosive devices, such as those used in mining, oil drilling, seismology and the like, especially shaped charges, and particularly shaped charges with a directional projection, and more particularly to shaped charges which may be used as perforators in the mining and petroleum industries.
2. Background of the Art
The history of shaped charge conception and development is imprecise. In 1792, the mining engineer, Franz von Baader (1792) allegedly noted that one can focus the energy of an explosive blast on a small area by forming a hollow in the charge. It is said that Baader, in 1799, observed that if depressions or shapes were cut in an explosive and placed face down on a steel plate, the detonation would cause these shapes to appear on the plate. This is known as explosive engraving.
The original von Baader (1792) paper, however, primarily discussed bore hole drilling and loading, confinement effects on propellants, the positioning of a small air cavity between the explosive powder and the tamping (at best, a standoff distance effect), and rock fragmentation. His original paper did not discuss explosive engraving or hollow cavity charges. However, this is a moot point since von Baader used black powder, which is not capable of detonation or shock formation.
During World War II, many new military explosives and explosive devices were introduced. One impressive device was the lined-cavity shaped charge. This type of explosive charge was the basis for the effect of the bazooka against metallic armor and for the deep penetration of shaped demolition charges when fired against reinforced concrete.
After the war, attempts were made to put the extraordinary penetrating power of the lined-cavity shaped charge to use in industry, particularly in the mining industry. These attempts were particularly successful in the petroleum industry where the use of shaped charges has become commonplace. On the other hand, the use of shaped charges in the mining industry is virtually nonexistent.
The most desirable shaped charge devices, such as penetrators, use the most efficient and the most powerful explosives, both for the benefits of raw power in the explosion and the speed of the detonation reaction which plays a role in the physics of the jet and the propelled mass. The use of these types of materials is quite hazardous, both in on-site handling and in transportation to the site. Even though the materials used in the manufacture of the penetrators may be relatively inexpensive relative to the volumes used, and the actual manufacturing costs may be relatively low, there are significant additional costs involved in safety procedures during handling and shipping (including insurance) which add a significant component to the final costs, without any benefit to the manufacturer or user.
U.S. Pat. No. 5,479,860 describes an apparatus for perforating an earth formation from a borehole having a housing, a detonator assembly, explosive material for producing the implosion forces and a metal liner of implosive geometry. The detonator assembly includes a predetermined pattern of precision electronic detonators based on exploding foil initiator, exploding bridge wire, spark gap or laser technology to simultaneously produce multiple initiation points of the explosive material for enhancing the implosion forces.
U.S. Pat. No. 4,693,181 discloses improvements in hollow charges for linear cutting or demolition purposes wherein a bar formed from a composite of explosive material and a first pliant material has a V-shaped groove with a liner formed from a composite of particulate metal and a second pliant material. The metal may be copper and preferably the first and second pliant materials include the same constituents. The charge may include a casing having a spacing portion having an engagement surface for presentation to a work surface, which engagement surface is parallel to the outer edges of the liner and spaced therefrom to maintain an optimum stand-off distance. The casing may further include a groove filling portion of low density material which may be integrally constructed with the casing from a flexible material such as expanded polyethylene.
U.S. Pat. No. 4,762,067 discloses a perforating gun detonator. One embodiment is hermetically sealed while the other has openings therein to admit well fluids. In both embodiments, a narrow conductive metal foil is provided with a current to vaporize the narrow foil, explode the foil and propel a flyer driven by a shock wave for detonation of a spaced secondary explosive. The explosive then couples explosion into a detonating cord against a shoulder in a housing adjacent to the secondary explosive. The current is formed by means of an AC voltage multiplier circuit providing a charge on a capacitor which is discharged through a spark gap. Charging circuitry includes a blocking capacitor to prevent DC and a resistor for bleeding a small current from the capacitor to ground which prevents static or stray current accumulation.
U.S. Pat. No. 4,498,3678 describes determination of parameters for selecting materials for use as liners in shaped charges to transfer the greatest amount of energy to the explosive jet. Multi-layer liners constructed of metal in shaped charges for oil well perforators or other applications are selected in accordance with the invention to maximize the penetrating effect of the explosive jet by reference to four parameters: (1) Adjusting the explosive charge to liner mass ratio to achieve a balance between the amount of explosive used in a shaped charge and the areal density of the liner material; (2) Adjusting the ductility of each layer of a multi-layer liner to enhance the formation of a longer energy jet; (3) Buffering the intermediate layers of a multi-layer liner by varying the properties of each layer, e.g., composition, thickness, ductility, acoustic impedance and areal density, to protect the final inside layer of high density material from shattering upon impact of the explosive force and, instead, flow smoothly into a jet; and (4) Adjusting the impedance of the layers in a liner to enhance the transmission and reduce the reflection of explosive energy across the interface between layers.
U.S. Pat. No. 5,351,623 describes a device which safely simulates the loud noise and bright flash of light of an explosion. This device consists of an ordnance case which encloses a battery, an electronic control module, a charging circuit board, a bridge head, and a shock tube dusted with aluminum and an explosive. The electronic control module provides a time delay between initial activation of the device and the time when the device is ready to create a shock wave. Further, this electronic control module provides a central control for the electronics in the simulator. The charging circuit board uses the battery to charge a capacitor. Passing the voltage stored in the capacitor through the wires of the bridge head causes the explosive and the aluminum in the shock tube to react. This reaction produces a loud noise and bright white flash of light which simulates an explosion.
One other aspect of explosive devices which has been of great concern is the danger of premature detonation of the device or charge. The highly energetic release of the compositions used for providing explosions has usually been attended by a high degree of sensitivity or a low initiation threshold for the explosive reaction. Attempts at alternative energy sources for explosive devices have led in many directions, including the electrical ignition of metals in water. W. M. Lee, Metal/Water Chemical Reaction Coupled to a Pulsed Electrical Discharge, J. Appl. Phys. 69 (10), 15 May 1991 describes how capacitor stored energy is transferred to a wire conductor surrounded by a mixture of a reactive metal powder and water. The current explodes the small wire conductor and initiates a chemical reaction in the mixture. The chemical reaction in the mixture was direct reaction of the aluminum metal and the water as EQU 2Al+3H.sub.2 O.fwdarw.Al.sub.2 O.sub.3 +3H.sub.2
to provide the energy for the investigation of explosive sources.
T. G. Theofanous, X. Chen and P. Di Piazza, Ignition of Aluminum Droplets Behind Shock Waves in Water, Phys. Fluids 6 (11), November 1994, pp. 3513-15 describes the reaction of gram quantities of molten aluminum with water under sustained pressure pulses of up to 40.8 Mpa in a hydrodynamic shock tube. Conditions are identified under which the thermal interaction develops into chemical ignition and total combustion events in the aluminum-water explosion.
Electrically triggered explosive devices are not per se novel. Electrical current has been used for more than one hundred years to ignite detonators, as for example with TNT or dynamite charges. Electrical signals are also used with modem explosive devices, including Explosive Bridge Wires and their membrane equivalents. Explosive bridge wires are thin wire(s) placed adjacent to an explosive charge. The wire(s) or membranes (exploding foil initiators) are very thin and have very low mass relative to the total mass of the charge (considerably less than 1% by weight). These films or wire(s) are placed adjacent to the explosive mass, and are electrically connected to a charge generator. The charge causes the wire to burst, creating a shock wave into and through the explosive material which initiates or enhances the explosive effect of the charge. Explosive charges are generally oxygen deficient, but of course, may still react with the burst wire or foil in a redox reaction.